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Fix derived-type macro dispatch capping members at 62 instead of 63

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NLOHMANN_JSON_GET_MACRO resolves 64 positional arguments, with NAME at
position 65. NLOHMANN_JSON_TYPE_TAG dispatches on Type plus the member
list, so it resolves correctly up to the 63 members NLOHMANN_JSON_PASTE
supports. NLOHMANN_JSON_DERIVED_TYPE_TAG dispatched on the two-token
Type,BaseType prefix plus the member list, running out one slot early:
at 63 members, position 65 landed on the last member name instead of a
sentinel and NLOHMANN_JSON_CAT built an undefined identifier such as
NLOHMANN_JSON_DEFINE_DERIVED_TYPE_INTRUSIVE_m63, with the compiler
reporting "unknown type name 'm1'" once per member and nothing pointing
at an argument-count limit.

That silently reduced all six NLOHMANN_DEFINE_DERIVED_TYPE_* macros from
63 members to 62, contradicting the "up to 63 members" contract in
docs/mkdocs/docs/api/macros/nlohmann_define_derived_type.md.

Drop the leading Type and defer to NLOHMANN_JSON_TYPE_TAG so the tag is
computed from BaseType plus the member list, which fits the available
slots. The zero-own-member derived bodies are therefore selected by tag
1 rather than 2, and the sentinel table for the derived tag is no longer
needed.

Add a regression test at the documented maximum for both the plain and
the derived macros; it fails to compile against the previous dispatch.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-19 21:03:08 +02:00
parent 4dcd45b26d
commit 0b28c63bad
3 changed files with 97 additions and 36 deletions
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tests/src/unit-udt_macro.cpp
+63
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@@ -896,6 +896,36 @@ class empty_derived_non_intrusive_only_serialize : public person_with_private_da
// NOLINTNEXTLINE(misc-use-internal-linkage)
NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(empty_derived_non_intrusive_only_serialize, person_with_private_data)
// Types at the documented maximum member count (63) for issue #4041's
// argument-count dispatch. The derived-type macros carry a two-token
// Type,BaseType prefix, so they reach two slots further into
// NLOHMANN_JSON_GET_MACRO than the non-derived ones and are the first to break
// if the tag dispatch runs out of positional slots.
class max_members
{
public:
int m1{}, m2{}, m3{}, m4{}, m5{}, m6{}, m7{}, m8{}, m9{}, m10{}, m11{}, m12{}, m13{}, m14{}, m15{}, m16{}, m17{}, m18{}, m19{}, m20{}, m21{}, m22{}, m23{}, m24{}, m25{}, m26{}, m27{}, m28{}, m29{}, m30{}, m31{}, m32{}, m33{}, m34{}, m35{}, m36{}, m37{}, m38{}, m39{}, m40{}, m41{}, m42{}, m43{}, m44{}, m45{}, m46{}, m47{}, m48{}, m49{}, m50{}, m51{}, m52{}, m53{}, m54{}, m55{}, m56{}, m57{}, m58{}, m59{}, m60{}, m61{}, m62{}, m63{};
NLOHMANN_DEFINE_TYPE_INTRUSIVE(max_members, m1, m2, m3, m4, m5, m6, m7, m8, m9, m10, m11, m12, m13, m14, m15, m16, m17, m18, m19, m20, m21, m22, m23, m24, m25, m26, m27, m28, m29, m30, m31, m32, m33, m34, m35, m36, m37, m38, m39, m40, m41, m42, m43, m44, m45, m46, m47, m48, m49, m50, m51, m52, m53, m54, m55, m56, m57, m58, m59, m60, m61, m62, m63)
};
class max_members_base
{
public:
int base_value = 0;
NLOHMANN_DEFINE_TYPE_INTRUSIVE(max_members_base, base_value)
};
class max_members_derived : public max_members_base
{
public:
int m1{}, m2{}, m3{}, m4{}, m5{}, m6{}, m7{}, m8{}, m9{}, m10{}, m11{}, m12{}, m13{}, m14{}, m15{}, m16{}, m17{}, m18{}, m19{}, m20{}, m21{}, m22{}, m23{}, m24{}, m25{}, m26{}, m27{}, m28{}, m29{}, m30{}, m31{}, m32{}, m33{}, m34{}, m35{}, m36{}, m37{}, m38{}, m39{}, m40{}, m41{}, m42{}, m43{}, m44{}, m45{}, m46{}, m47{}, m48{}, m49{}, m50{}, m51{}, m52{}, m53{}, m54{}, m55{}, m56{}, m57{}, m58{}, m59{}, m60{}, m61{}, m62{}, m63{};
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(max_members_derived, max_members_base, m1, m2, m3, m4, m5, m6, m7, m8, m9, m10, m11, m12, m13, m14, m15, m16, m17, m18, m19, m20, m21, m22, m23, m24, m25, m26, m27, m28, m29, m30, m31, m32, m33, m34, m35, m36, m37, m38, m39, m40, m41, m42, m43, m44, m45, m46, m47, m48, m49, m50, m51, m52, m53, m54, m55, m56, m57, m58, m59, m60, m61, m62, m63)
};
} // namespace persons
TEST_CASE_TEMPLATE("Serialization/deserialization via NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE", Pair, // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
@@ -1414,3 +1444,36 @@ TEST_CASE_TEMPLATE("Serialization/deserialization of zero-member types via NLOHM
CHECK(j.dump() == derived_dump);
}
}
// Regression test for the argument-count dispatch added for issue #4041: the
// documented maximum of 63 members must keep working, including for the
// derived-type macros whose Type,BaseType prefix consumes two dispatch slots.
TEST_CASE_TEMPLATE("Serialization/deserialization of maximum-member-count types via NLOHMANN_DEFINE_TYPE_*", Json, // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
nlohmann::json, nlohmann::ordered_json)
{
SECTION("NLOHMANN_DEFINE_TYPE_INTRUSIVE with 63 members")
{
persons::max_members obj{};
obj.m1 = 1;
obj.m63 = 63;
Json j = obj;
CHECK(j.size() == 63);
const auto obj2 = j.template get<persons::max_members>();
CHECK(obj2.m1 == 1);
CHECK(obj2.m63 == 63);
}
SECTION("NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE with 63 own members")
{
persons::max_members_derived obj{};
obj.base_value = 7;
obj.m1 = 1;
obj.m63 = 63;
Json j = obj;
CHECK(j.size() == 64);
const auto obj2 = j.template get<persons::max_members_derived>();
CHECK(obj2.base_value == 7);
CHECK(obj2.m1 == 1);
CHECK(obj2.m63 == 63);
}
}